Ruthenium (Ru) is expected to be introduced in the industrial semiconductor manufacturing process for many applications in the coming years. This move towards the use of new materials for chip manufacturing is necessary to solve issues generated by the continuous scaling trend imposed to the industry. For the next generation nodes, Ru is considered as the best candidate for the electrode capacitor for FeRAM and DRAM applications. Ru has the required properties, such as high melting point, low resistivity, high oxidation resistance and adequate work function, making it a potential gate electrode material for CMOS transistor. Ru has advantages compared to iridium and platinum due to its lower resistivity and Ru ease of dry etching. Additionally, RuO2 has a high conductivity so the formation of Ru oxide by diffusion of oxygen, that could come from ferroelectric films (PZT, SBT, BLT, . . . ), will have less impact on electrical properties than other metal oxides known to be more insulating.
Ru is also a promising BEOL process candidate as a liner material for copper. A single Ru film would replace the Ta, Ta(Si)N, and the copper seed layer. A Ru process replaces the two steps Ta/Ta(Si)N process with a single step.
A large variety of Ru CVD precursors are available and many have been studied. However, the currently available precursors have a very low vapor pressure (i.e. 0.1 Torr at 73° C. for Ru(EtCp)2). The Ru films obtained with these known precursors contain significant amounts of carbon and oxygen impurities. The C impurities are suspected to come from the precursor material. The O impurity comes from the co-reactant gas (O2). It is known from T. Shibutami et al, Tosoh R&D Review, 47, 2003, that Ru films have poor adherence, uniformity and also have a characteristically long incubation time.
It is known from e.g. U.S. Pat. No. 6,897,160, to use Ru precursors such as tricarbonyl (1,3-cyclohexadiene) Ru precursor to deposit rough ruthenium oxide layers, wherein said particular precursor (see example 4 of said patent) is held in a bubbler reservoir at room temperature (about 25° C.) and helium is bubbled through it.
However, as explained in U.S. Pat. No. 6,517,616 to the same assignee, such Ru(CO)3(1,3-cyclohexadiene) product is not liquid at room temperature (it melts at about 35° C.) and it is necessary to dissolve such precursor in a solvent in order to obtain a liquid solution of precursor and solvent through which the inert gas such as helium is bubbled (U.S. Pat. No. 6,517,616 discloses in great details to ruthenium precursors solutions in solvents—see also U.S. Pat. No. 5,962,716).
Ru(CO)3(1,3-cyclohexadiene) is disclosed also in the article of Y. W. Song et al., ECS transactions, 2(4), 1-11, 2006, including its solid nature at ambient temperature, with a melting point of 35° C.
All the known precursors of Ru containing a CO molecule have essentially the same drawbacks which is their high melting point. such solid products require the use of a solvent in order to handle a liquid product through which can be delivered into the reactor either through direct vaporization or by bubbling an inert gas through it.
However, the use of a solvent is usually viewed as having a bad influence on the deposition process due to the intrusion of the solvent particles in the reactor and the incorporation of undesired impurities in the deposited films. Moreover, the solvents used are usually toxic and/or flammable and their usage brings many constraints (safety aspects, environmental issues). Besides, the use of precursors with melting points higher than 20° C. (and even for those having a melting point above 0° C.) implies many additional constraints during the process deposition (heating of the delivery lines to avoid condensation of the precursor at undesired locations) and during the transportation.
The reactivity of the known CO containing precursors does not enable today to reach an ALD deposition regime. Ruthenium films are only deposited by CVD and some articles even outline that ALD mode is not possible with the Ru(CO)3(1,3-cyclohexadiene) precursor.